The long term objective of this research proposal is to understand how cerebellar granule neurons induce dendritic differentiation of Purkinje cells. The experiments described here use a novel culture system comprised of purified murine granule and Purkinje cells, alone or together in vitro. This purified, in vitro approach eliminates the confounding influence of many other cell types that have been present in previous in work. The defined cell culture system allows testing of various agents for effects specifically on the Purkinje cell. One set of proposed experiments constitutes a physical/biochemical approach that makes no assumptions a priori about the nature of the granule neuron signal for Purkinje cell differentiation. A second set of experiments represents the "candidate gene" approach, in which neurotrophins, growth factors, or depolarizing agents previously suggested to affect neuronal development are tested for their expression and effects during granule neuron-induced Purkinje cell differentiation. These studies will identify cues that trigger cerebellar neuronal development, and may reveal new agents with therapeutic value in treating neurodegenerative diseases of the cerebellum.